William starts counting at 31, and he counts by fours. If 31 is the 1st number that William counts. what is the 13th number that he counts?
Explanation: What is the first number that he counts? $31$ What is the second number that he counts? $\begin{align*}&31 + 4 \\ &= 35\end{align*}$ What is the third number that he counts? $\begin{align*}&31 + 4 + 4 \\ &= 31 + (2 \times 4) \\ &= 39\end{align*}$ What is the 13th number that he counts? $\begin{align*}&31 + (12\times4) \\ &= 31 + 48 \\ &= 79\end{align*}$